Chiral hafnocenes are useful for the synthesis of polyolefins. The racemic form of these hafnocenes provides stereoregular poly(alpha-olefins). In addition, the racemic form of these hafnocenes is considerably more catalytically active than the meso form, which produces only atactic polymers. Chiral hafnocenes and their use as catalysts in forming isotactic olefin polymers are described, for example, in U.S. Pat. Nos. 4,794,096; 5,017,714; 5,036,035; 5,036,034; 5,145,819; 5,296,434; 5,324,800; 5,329,033; 5,399,636; 5,455,365; 5,455,366; 5,532,396; and 5,556,997, the full disclosures of which are incorporated herein by reference.
It has been discovered that although hafnium is a member of Group 4, in the synthesis of metallocenes hafnium reactants can behave in a remarkably different manner from corresponding titanium or zirconium compounds. In particular, as shown by my colleagues, S. P. Diefenbach, M. S. Ao, J. M. Power and J. R. Strickler in U.S. Pat. No. 5,302,733, ZrCl.sub.4 readily forms a tetrahydrofuran complex ZrCl.sub.4 (THF).sub.2 by addition of THF to slurry of ZrCl.sub.4 in a medium such as methylene chloride or hexanes. Moreover the ZrCl.sub.4 (THF).sub.2 complex can be reacted in tetrahydrofuran with lithium derivatives of various silicon-containing cyclopentadienyl ligands such as the dilithio derivative of 1,1'-dimethylsilanylenebis(indene)ethyl etherate to produce racemic bridged zirconocenes such as rac-1,1'-dimethylsilanylenebis(indenylzirconium dichloride. In fact this reaction was successfully performed at reaction temperatures between 50 and 54.degree. C. (note Example 3 of U.S. Pat. No. 5,302,733). In sharp contrast, if the same reaction is attempted with HfCl.sub.4, copious amounts of tar are produced and the yield of isolatable racemic isomer of the bridged hafnocene, so low as to be practically non-existent. Furthermore, the zirconocene is stable in THF solution whereas the hafnocene is very unstable in THF solvent even at room temperature.
A useful contribution to the art would be the provision of new process technology for producing racemic bridged hafnocenes, in higher yields and with better product quality at lower cost. This invention is deemed to constitute such a contribution.